Signal enhancement circuit

ABSTRACT

A synchronous oscillator demodulator for linear variable differential transformers and synchronous resolvers having electronic circuitry designed to digitally construct a preferred waveform of a predetermined frequency and to establish precisely the preset point at which the amplitude of the waveform is to be measured at the secondary winding of the transformer. This synchronous oscillator demodulator permits the use of linear variable differential transformers and synchronous resolvers for high frequency applications and under conditions of noise and vibration while still maintaining a high degree of accuracy and repeatability. A system and apparatus to enhance the signal from a given linear variable differential transformer and tune sane in relation to the signals of other similar but not identical transformers is disclosed.

BACKGROUND OF THE INVENTION

This invention pertains to the input signal for the primary windings ina linear variable differential transformer hereafter LVDT or asynchronous resolver hereafter SR. Such devices are readily availableand are disclosed in the early prior art, see, for example, theMacgeorge U.S. Pat. No. 2,427,866 covering an LVDT wound withsymetrically spaced identical secondaries adjacent to a central primaryon a common coil form wherein the core is longer than the primarywinding. Similarly, a synchronous resolver is shown in the SchaevitzU.S. Pat. No. 2,494,493 wherein the coil form contains a pair ofidentical windings, and spaced symmetrically above and below the middleand within the coil is a core being a cardioidal shaped magneticpivotally and eccentrically mounted on an input shaft positioneddiametrically across the coil.

Linear movement of the core (LVDT) or rotation of the shaft (SR)influences the phase and amplitude of the waveform generated in thesecondary in relation to the amount of movement or rotation.

The cross-section of a typical LVDT consists of three symmetricallyspaced coils a primary and a pair of secondaries connected in seriescarefully wound about an insulated bobbin and four wire leads exitedthrough one end. An outer shield of a ferro-magnetic material is placedover the windings which are vacuum impregnated with a suitable pottingcompound. Consequently, the finished transformer is impervious tohumidity and magnetic influences. The core is made of a uniformly densecylinder of nickel-iron alloy which is annealed to improve itshomogeniety with respect to magnetic permeability. The LVDT is africtionless device since there is no physical contact between themovable core and the LVDT coil structure. The absence of friction andcontact between the coil and the core of an LVDT means that there isnothing to wear out and gives the LVDT an essential infinite mechanicallife.

The nominal linear range of travel of an LVDT is the distance the coremay be displaced in either direction from its null position. Thesymmetry of the LVDT construction provides null point repeatability.More particularly, the LVDT produce an electronic output proportional tothe placement of the movable core. A waveform excitation applied to theprimary induces a similar excitation in the secondary. The two identicalsecondaries are symmetrically spaced from the primary and adjacentthereto in axial relation therewith on each side thereof. Thesecondaries are connected in a series opposing circuit. As the motion ofthe noncontacting magnetic core varies, the magnetic inductance of eachsecondary relative to the primary is thereby determined by the inducedvoltage difference. If the core is moved off center, the magneticinductance of the primary with respect to one secondary will be greaterthan with respect to the other and a differential voltage will appearacross the secondary output terminals. For offset displacements withinthe normal operating range, the voltage is a linear function withrespect to displacement with some deviation due to tolerances infabrication of the LVDT.

When the middle of the core is centered between the secondary windings,i.e., is at the center point of the primary winding, the voltage inducedin each secondary is equal and 180° out of phase so there is nosecondary output.

Certainty of waveform shape and frequency is absolutely essential toproper measurement of small differences in linear displacement at highperiodicities. Therefore, uniform waveforms with a frequency greaterthan that of the oscillation of the core are required for accuracy andrepeatability. The use of 60 Hz power line frequency for excitation ofthe primary coil is acceptable for core oscillations under six cyclesper second. The rule of thumb is that the excitation frequency must beat least ten times greater than the highest modulation frequency to bemeasured as a component of mechanical motion.

The majority of LVDT applications apply sine waveforms to the primarycoil which waveforms should be free from harmonic distortions sincemodulated distortion may increase the null voltage. Excessive nullvoltage requires filtering the excitation voltage and/or the LVDT outputto remove harmonics which affect the accuracy in connection withmeasuring small displacements at high frequencies. Moreover, thevariations in output from one LVDT to another require a means by whichthe LVDT's can be equalized and their deviation from linearity can beminimized to a point where it is negligible.

The improvement of the present invention will provide a uniform highfrequency waveform and a means to specify the point on said waveformwhere the amplitude of the output from the secondaries will be used.Similarly, the output from the secondaries of any given LVDT will becorrected to overcome any deviations due to particular characteristicsof the LVDT relative to an ideal LVDT with totally linear response.

OBJECTS OF THE INVENTION

It is an object of the invention to provide a waveform generator circuitwhich will enable an LVDT to measure high frequency displacements.

It is a further object of the invention to provide an electronic circuitwhich will permit each of a plurality of LVDT's to be tuned andcalibrated so that they measure small displacements at high frequenciesaccurately and repeatably.

There is yet another object of the invention to provide a readout systemwhich will give a permanent and quickly interpreted display of themeasurements obtained by the LVDT and electronic circuitry.

It is still another object of the invention to provide an accurate,reliable, stable and repeatable system for measuring the high frequencyoscillations a travel range which may vary less than 1/10,000 of aninch. The foregoing objects and problems of the prior art will beaccomplished and overcome respectively by electronic circuitry and itsadaptation for use with LVDT's.

SUMMARY OF THE INVENTION

A waveform generator circuit is disclosed and includes electronicmicro-circuitry for counting at a prescribed rate. The counting takesplace at a rate sufficiently high to be used for generating a waveformappropriate for the excitation rate necessary to drive the primary of anLVDT used for measuring changes in linear distance which takes place atan oscillation frequency of several hundred per minute. Moreparticularly, the frequency of 125 strokes per minute as disclosed inU.S. patent application Ser. No. 948,714 now U.S. Pat. No. 4,213,319 fora thickness gauge must be measured by a waveform with a frequency of atleast 1250 cycles per minute. The preferred embodiment of the disclosureherein is for use in connection with that thickness gauge and requireshigh accuracy in an environment of great noise and vibration. Thetransducer in that application are the LVDT from which the signalemanates and for which signal conditioning and enhancement as disclosedherein is required.

The enhancement circuit calculates the thickness reading from a givenLVDT voltage signal and in doing so minimizes any individual undesirablecharacteristics of that LVDT to the point where such characteristics areignorable. Multiple LVDT transducers may be used to measure variouspositions across a skeleton web in the preferred embodiment withoutconcern for tolerance differences between them. More specifically, thepresent circuit permits the LVDT to be used in the range of its maximumlinearity and overcomes the voltage output. differences relative to coretravel. The unique characteristics of each LVDT are thereby accountedfor and the voltage signals are converted to relevant units of measureto an accuracy of more than 0.0001 inches.

The data so obtained and transformed is stored in a computer until thereadout is required; at that time a tabular format for the printout orcathode ray tube display is used. The tabular format is constructed topermit prompt trend interpretation and out of tolerance isolation byrelatively unskilled personnel. More specifically, the position of thedata in the table is directly related to the amount by which thetolerance of the thickness is out of specification. Consequently, a rowof data will shift with the trend and odd data will immediately standout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a substantially schematic circuit diagram showing theelectronic components for conditioning a signal to be amplified for theLVDT input;

FIG. 2 is a series of schematic timing diagram illustrative of thesignals in the signal conditioning circuit;

FIG. 3 is a schematic of the preferred embodiment and its relationshipwith the input and output apparatus, and

FIG. 4 is a graphic showing of the formula which is used to enhance theoutput signal from the single conditioning circuit.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, an electronic circuit is shown for developing a preferredwaveform of a specific frequency. The preferred waveform is a sine waveand in order to generate a sine wave of an appropriate frequency for usewith the output from the thickness sensors hereinbefore described, thereis a clock generator which presents a continuous series of periodicpulses each of which are 0.5 μs in duration. A crystal oscillator isused to set the frequency period. The clock pulses are transmitted to acounter which consists of a series of flip-flop circuits contained in 3,4-bit binary counters. While 12-bits are available only 10 are used forthe preferred embodiment. The flip-flops determine how many 0.5 μspulses are passed into the counter. 1024 pulses are counted within1/2000 of a second. The counter is arranged to note when 1024 countshave been made and begin counting from zero once again. Consequently,the counter is continuously incremented and recirculated when driven bythe clock pulses from the clock generator. The counter has 12connections to a read only memory chip. The 12 connections from thecounter are used to represent the numbers 1 through 1024 in the binarynumbering system. Consequently, an individual signal is given to theread only memory from the counter for each count, and such signals arespecifically entered in the read only memory. Also connected to thecounter by 12 lines is an address decoder which is set to generate anoutput pulse once every cycle of the sine wave. This pulse is typicallygenerated at the peak of the sine wave in the secondary windings. Byselecting the decoder pulse it is thus possible to compensate forpossible phase shift of LVDT or SR. The phase shift can be determined byusing an oscilloscope. More particularly the timing is arranged tocoincide with the peak of a sine wave to be generated by the read onlymemory. That is, data is stored in the read only memory which gives theamplitude of a sine wave signal at each of the counts from the counter.Therefore, the sine wave can be numerically generated. Moreparticularly, the read only memory is programmed with the data that canbe fed into a digital-to-analog converter to generate a sine wave. Eachcount is used to provide a timing input to trigger the stored dataoutput of the specific amplitude. In the preferred embodiment 1024points of amplitude data are generated to specify the sine function withrespect to time.

The amplitude values in binary form are transmitted by 12 data linesfrom the read only memory to a digital-to-analog converter. Morespecfically, a digital-to-analog converter translates the binaryamplitude number into a measurable voltage which varies with changingamplitude to generate the sine wave. The mathematically constructedanalog signal represents a sine wave of the specified preset frequencyand it is amplified and transmitted to the primary coil of an LVDT or anSR. The secondaries of the LVDT inductively pick up the amplified sinewave and in accordance with the position of the LVDT core, transmit theinduced signal to a sample and hold circuit.

The sample and hold circuit memorizes the amplitude timing of the sinewave from the secondary windings and sampling pulses transmitted overper cycle relative to the digitally constructed sine wave, from theaddress decoder are used to trip the sample and hold signal at the pointwhere the amplitude of the secondary sine wave is to be read.Consequently, the secondary output is released at a prescribed amplitudepoint per the signal from the address decoder.

Another means by which the sine wave for the primary of the LVDT can begenerated is through the use of a microprocessor instead of the clockgenerator and counter. Such a microprocessor would be programmed with analgorithm to determine the particular analog function to be generatedi.e., a sine wave, a triangular waveform or any other regular function.Such programming would also cause the sample pulse to occur at apreselected time during the particular function. Therefore, as theprogram in the read only memory is executed by the microprocessor, datais transmitted to the digital-to-analog converter whereby an analogoutput is available for amplification to drive the LVDT or SR. The restof the circuit is essentially the same. Yet another approach would be toreplace the counter with an up/down counter which would reverse itscounting direction when it reached the preset limit and thereby countback to zero and reverse again. Such an approach could be used byconnecting directly to the digital-to-analog converter which wouldgenerate an analog output voltage to represent the continuouslyincremented and deincremented counting of the up/down counter.Typically, a triangular waveform would be generated at the rate ofcounting. Once again a sample pulse decoder would specify the point atwhich the function would be read at the output side of the secondariesof the LVDT. The decoder is the same as described before and itgenerates the sampling pulse.

FIG. 2 shows the schematic timing diagrams for the preferred embodiment,more specifically when the core of the LVDT is located within thethickness gauge. Three thickness gauges are attached to the ram of ametal working press and move up and down therewith at a rate ofapproximately 125 strokes per minute. As each thickness gauge is broughtto bear across the skeleton whereby a portion of the gauge rests on thepress platten beneath the skeleton and another sensor portion bears uponthe scrap. The relative difference between the portions is a measure ofthe scrap thickness. That difference is periodically transmitted to thecore of the LVDT as shown in FIG. 2. The core changes position withrespect to time in that it is first moving with the press ram and thenit is moving with respect to the press ram. As it is displaced by thescrap thickness, it is moving with respect to the press ram. The strokeof the press ram is sinusoidal (FIG. 2) and the thickness gauge isdesigned to overstroke relative to the stroke of the press ram since theportions therein are resiliently mounted. The sinusoidal time diagramfor the thickness gauge shows the press ram motion relative to thesensor portion during contact with the coil stock. More particularly, onstroke "N", the sensor is in contact with the stock for a period (55 ms)greater than just the instant at which the press ram reaches the bottomof its stroke and begins to return. Similarly, the sensor contacts thecoil and overstrokes on the press stroke "N+1". Overstroking isnecessary since the output from the LVDT core rod movement duringoverstroking initially includes oscillations at the instance of contactwith the coil stock. Such core oscillations diminish rapidly during theoverstroke and a point is reached whereat a stable and repeatable signalcan be taken from the LVDT (see FIG. 2) and that signal is a reliablemeasure of the stock thickness.

A synchronized pulse having a period commensurate with the timing of thestable, repeatable, fully damped output from the LVDT is generated byany convenient means, e.g. a switch connected to the press ram or aclock generator and is used to specify the point during the overstrokeat which the thickness reading defined by the core displacement will betaken. The LVDT primary is driven with a sine wave having an amplitudeof over 2000 cycles per second. Consequently, the secondary output ofthe LVDT is over 2000 cycles per second. In addition to that, the outputof the secondary windings will peak at known points in accordance withcycling and the signal conditioning as shown in FIG. 1 and hereindescribed. It is, therefore possible to select points by means of asample and hold circuit at which the secondary output will be used tomeasure the thickness. In essence, such a sample and hold circuitgenerates a digitized signal giving a definite peak at which the outputcan be monitored as a voltage representative of the thickness of thecoil stock. At any instant such a voltage is a measurement received fromone of the three thickness gauges.

In FIG. 3, there is a schematic showing of how the aforesaid voltagesignal is generated by the three thickness sensors and transmitted tothree LVDT signal conditioners which convert the signals to three outputvoltages at any instant during which readings are taken. These threeoutput voltages are transferred to a microprocessor for enhancement andstorage. Part of the storage function is to arrange the outputs inseriatum fashion so that it is known from which thickness sensor aparticular output arrived. The microprocessor is designed to take eachsignal and decide from which thickness sensor it was derived and thentune that output voltage in accordance with the particularcharacteristic from the measuring LVDT of its thickness sensor. Moreparticularly, and as discussed herein, each LVDT has its owncharacteristic which must be accounted for in order to tune each outputsignal and equalize it relative to the other output signals from theother thickness sensors. The microprocessor is programmed with a formulato be used to convert and tune each.

In FIG. 4, the dashed line represents the true output from an individualLVDT and the straight line with the equation:

    y=mx+b

will pass through y₂ and y₁ intercepting the y axis at "offset" voltage"b". The slope of the straight line is "m" (expressed in volts per inch)and it can also be called the "scale factor". The LVDTs rarely have thesame null voltage or scale factor because of the difficulty of makingall secondary windings identical. More specifically, it is impossible tomake all LVDT windings of the same performance values and accuracy. Inthe thickness gauge application the accuracy of thickness readings mustbe in the range of 0.0090" to 0.0130. The idealized straight line foreach of the three LVDT sensors used is established using gauge plates.Each plate measure 13/4"×5" and is perforated with 33/4" diameter holesto permit passage of the outer three pins of the outer coaxial plungeron each sensor. Thickness in the triangular area can be measured to thenearest 0.00001" by conventional means (Jo blocks, comparator, etc.),and to calibrate the press is jogged to the bottom dead center positionwhere the output voltage corresponding to the known thickness gaugeplates are recorded. In this manner a voltage y₁ corresponding to aknown thickness x₁ and a voltage y₂ corresponding to known thickness x₂can be found. Thus, multiple readings can be taken to minimize thepossibility of errors due to reading inaccuracy or variations due tomechanical tolerances. As a further check, static readings from theactual coil stock are taken and the scrap skeletons are retrieved forlater measurement.

The scale factor "m" is the slope of a straight diagonal line in FIG. 4,and the numerical value of "m" can be calculated from the known valuesof voltage and thickness:

    m=(y.sub.2 -y.sub.1)/(x.sub.2 -x.sub.1).

For a given sensor, this establishes its scale factor based on a voltagewhich includes the effects of AC excitation, LVDT characteristics, andamplifier gain. With "m" known, the offset voltage, "b" can becalculated for any thickness

    x.sub.1 as (b=y.sub.1 -mx.sub.1).

It follows that for any thickness x_(n), within the linear range of thesystem, the desired thickness can be found from the equation

    x.sub.n =(yn-b)/m.

This is the relationship used by the microprocessor, FIG. 3, to convertand enhance an electrical signal to a thickness reading for a givensensor. Expressed in words, the thickness (being sensed) is equal to theDC output voltage minus the offset voltage divided by the scale factorfor the given sensor.

The foregoing procedure enhances the output of the LVDT and overcomesthe variations due to LVDT construction parameters. More particularly,there can be nonuniform wire cross-section or insulation and/ornonuniform winding of the coil (turns/cross-sectional inch). Similarly,coil strain due to packaging in the protective metal outer shell candistort carefully wound coils changing the scale factor or the offset.Maximum deviations from an idealized straight line relationship usuallyoccur near the null position or near the ends of the "linear range".LVDT vendors' linearity specifications are optimized to make them lookas good as possible over the entire useful range of displacement oftheir LVDT's and that makes them interchangeable and affordable. Iftheir specifications are not good enough for the application, the buyermust pay extra to get better specifications. The limits of the state ofthe art of fabrication technology are only aided by weeding out thepoorly made units and selling the good ones; even so, the tolerances areuncertain and unacceptable for measuring in the range of the thicknessgauge application. The microprocessor enhancement circuit enables theuse of LVDT's which have different characteristics and therebyeliminates the cost of purchasing high priced, selected LVDT's whichessentially cannot provide accurate readings to 0.0001 of an inch. Theenhancement permits the use of any LVDT in the area within its rangethat has the best linearity. Even if the LVDT is nonlinear a series ofmeasurements with gauge plates can be used to plot actual performanceand thereby program the microprocessor to translate any voltage outputto that of a straight line function. In the preferred embodiment theLVDT accuracy is sufficient to approximate the core travel range havingbest linearity with a straight line.

The enhancement signals from the microprocessor in FIG. 3 representcorrected, tuned output voltages from the thickness gauge sensors. Theseoutputs are fed seriatum to a computer programmed to convert the data tolinear measure and tabulate same. The computer is connected to a cathoderay tube for instantaneous readout at any given convenient location. Thecomputer is also connected to a printer so that the data may beaccumulated and recorded for later analysis. The format for thetabulation is such that the data is printed in three columns. Eachcolumn is associated with a particular thickness gauge sensor, and eachcolumn has a tolerance range which is visually depicted by shifting thedecimal point and the number associated therewith to the left or rightin that column, should the thickness in numerical value be greater orless than the ideal respectively. That is to say that, as the stockthickness is measured thinner than a predetermined ideal, the entirethickness reading is printed out or appears on the cathode ray tubeshifted to the right an amount equal to the tolerance difference. Bythis technique the column of data tends to weave in a wave-like mannerto the left and right in accordance with variations in the specifictolerances of the coil thickness. An unskilled operator can immediatelyperceive that the stock is somewhat out of specification, occassionallyout of specification or completely out of specification. The computer isalso programmed to tabulate and summarize the mean and average thicknessabout every two minutes whereby a summary of the material fed throughthe press can be monitored.

While a particular application has been shown for the excitation of anLVDT and for the enhancement of the signal emanating from the LVDT andfor particular print out of the signal so obtained, it is believed thatthose skilled in the arts which pertain to these devices will appreciatechanges which will make this applicable for use with an SR and othersimilar waveform driven devices. It is also desired that the enhancementtechnique of approximating a function with a known function and shiftingsame by rotation and translation as disclosed will be applicable apartfrom the particular application of thickness gauge sensing. Therefore,the claims which follow are meant to appropriately cover the inventionin its broadest aspects and most far ranging applications.

What is claimed is:
 1. A method for enhancing and adjusting a pluralityof variable signals from output devices of the same type each havingdifferent characteristics to overcome the pecularities of each and torelate and tune each to its maximum linearity over a preset rangeincluding the steps of:finding the most substantially linear portion ofeach of series of signals emanating from each device over said range byplotting the characteristics of each over said range, calibrating eachsaid device against a known straight line function to approximate thevariance of the output characteristic for each of said series of signalsrelative to said function, and correcting said signals for producing anoutput adjusted and enhanced by performing translation and rotation ofsaid straight line function to account for the peculiar characteristicof each said device.
 2. The method for enhancing and adjusting signalsof claim 1 wherein said output devices are transformers with excitedprimary windings and means by which the magnetic characteristics of sameare variable to alter the output of the secondary windings.
 3. Themethod for enhancing and adjusting signals of claim 2 wherein each ofsaid transformers is an LVDT.
 4. The method for enhancing and adjustingsignals of claim 1 wherein said enhancement is accomplished by anelectronic device arranged to correct the variable signals of a givenoutput device by changing same to represent said straight line.